Pipes used for oil, gas, and chemical transportation or as part of a refinery or processing facility may be covered by coating that can be over four inches thick. Some coatings provide thermal insulation to minimize heat transfer between the surroundings and the hot or cold materials inside of the pipe. Other coatings are applied to decrease the buoyancy of the pipe, such as a concrete weight coat for underwater pipes, or to protect the pipe from mechanical damage.
To protect thermal insulation from mechanical damage, an aluminum or stainless steel weather protection (or sometimes called a “weather jacket”) may be secured over the insulation and held in place by metal straps along the length of the pipe.
FIG. 1A shows a cross-section of a pipeline 100 which has a pipe 101, insulation 103, and weather protection 105. The relative dimensions of pipe 101, insulation 103 and weather protection 105 are illustrative. Actual pipes may have a wide range of parameters. Dimensions of note include the pipe's outer diameter, OD, inner diameter, ID, and wall thickness, ts (ts=[OD−ID]/2). The insulation has a thickness, ti, and the weather protection has a thickness, tw.
Over time, pipe 101 can become corroded, reducing the integrity of the pipeline and increasing the risk of a catastrophic failure. Corrosion can result in localized reduction in the wall thickness of the pipe, general wall loss over large areas, or localized pits. Material loss caused by corrosion can occur on both the inner and outer surfaces of the pipe as illustrated by the exemplary interior corrosion loss 107 and exterior corrosion loss 109. Other defect types that may be formed are cracks 106, 108 (e.g. stress corrosion cracking), welding anomalies, hard spots (local increases in material hardness), and dents.
If damage becomes severe, pipe 101 may fail, for example, as a result of the pressure of the transported gas/liquid. Inspections are needed to identify locations of pipe that are likely to fail so that local replacement or repair of the pipe section can be performed preemptively avoiding pipeline failure.
Inspection can be performed from the interior or exterior of the pipe. For interior inspection, a pipe inspection gauge (PIG) is inserted into the pipe and wall thickness measurements are made using inspection techniques such as magnetic flux leakage (MFL) and ultrasound. As the PIG passes down the pipe, inspection data is recorded which can then be used to identify sections of the pipe requiring replacement or repair.
When the pipe diameter is small or the pipe was not constructed to allow for the use of a PIG, interior inspection may not be practical. The inspection may also be performed from the exterior. Conventional exterior inspection techniques require that the weather protection and insulation be removed from the pipe so that visual, ultrasonic, or another inspection method can be performed. Such an inspection typically takes a considerable amount of time to perform. Further, the insulation and weather protection typically must be replaced after the inspection adding to the expense of pipeline inspection.